In battery/power management applications, PMOSFETs (p-channel metal oxide semiconductor field effect transistors) can be used as high side switches for controlling charging and discharging loops. Since the PMOSFET is usually more expensive than an NMOSFET (n-channel MOSFET) with the same turn-on resistance, sometimes NMOSFETs are used as high side switches. If the NMOSFET is used as the high side switch in an electronic system, e.g., a battery operating system, a charge pump can be used in order to fully turn on the NMOSFET even when the battery voltage is relatively low. However, the voltage of the battery may vary during operation. Consequently, a driving voltage generated by the charge pump may be too high that may break down the NMOSFET when the battery voltage is relatively high. To solve this problem, an extra voltage clamp can be used to limit the driving voltage to a predetermined value. However, extra power will be consumed by the voltage clamp and therefore the power efficiency of the electronic system is degraded.